ccanlint: check examples actually mention thing they are demonstrating.

Brad suggested this after finding one such cut & paste in str:

rusty@vivaldi:~/devel/cvs/ccan/ccan/str$ ../../tools/ccanlint/ccanlint
Example: sections demonstrate appropriate function (examples_relevant): FAIL
/home/rusty/devel/cvs/ccan/ccan/str/str.h:64:Example for strcount doesn't mention it
Total score: 37/38

tools/ccanlint/tests/examples_relevant.c

+#include <tools/ccanlint/ccanlint.h>
+#include <tools/tools.h>
+#include <ccan/talloc/talloc.h>
+#include <ccan/str/str.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <limits.h>
+#include <errno.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <err.h>
+#include <string.h>
+#include <ctype.h>
+
+static void examples_relevant_check(struct manifest *m,
+				    bool keep,
+				    unsigned int *timeleft,
+				    struct score *score)
+{
+	struct ccan_file *f;
+	struct doc_section *d;
+
+	list_for_each(&m->h_files, f, list) {
+		list_for_each(get_ccan_file_docs(f), d, list) {
+			unsigned int i;
+			bool found = false;
+
+			if (!streq(d->type, "example"))
+				continue;
+
+			for (i = 0; i < d->num_lines; i++) {
+				if (strstr(d->lines[i], d->function))
+					found = true;
+			}
+
+			if (!found) {
+				score_file_error(score, f, d->srcline+1,
+						 "Example for %s doesn't"
+						 " mention it", d->function);
+			}
+		}
+	}
+
+	if (!score->error) {
+		score->score = score->total;
+		score->pass = true;
+		return;
+	}
+}
+
+struct ccanlint examples_relevant = {
+	.key = "examples_relevant",
+	.name = "Example: sections demonstrate appropriate function",
+	.check = examples_relevant_check,
+	.needs = "examples_exist"
+};
+
+REGISTER_TEST(examples_relevant);